The optical properties of metallic nanostructures are a topic of considerable scientific and technological importance. The optical properties of a metallic nanoparticle are determined by its plasmon resonances, which are strongly dependent on particle geometry. The structural tunability of plasmon resonances has been one of the reasons for the growing interest in a rapidly expanding array of nanoparticle geometries, such as nanorods, nanorings, nanocubes, triangular nanoprisms, nanoshells, and branched nanocrystals. The resonant excitation of plasmons can lead to large local enhancements of the incident electromagnetic field at the nanoparticle surface, resulting in dramatically large enhancements of the cross section for nonlinear optical spectroscopies such as surface-enhanced Raman scattering. The structural dependence of both the local-field and far-field optical properties of nanoparticles across the visible and near-infrared (NIR) spectral regions has enabled their use in a wide range of biomedical applications, an area of increasing importance and societal impact.
Metallic nanoshells, composed of a spherical dielectric core surrounded by a concentric metal shell, support plasmon resonances whose energies are determined sensitively by inner core and outer shell dimensions. This geometric dependence arises from the hybridization between cavity plasmons supported by the inner surface of the shell and the sphere plasmons of the outer surface. This interaction results in the formation of two hybridized plasmons, a low-energy symmetric or “bonding” plasmon and a high-energy antisymmetric or “antibonding” plasmon mode. The bonding plasmon interacts strongly with an incident optical field, whereas the antibonding plasmon mode interacts only weakly with the incident light and can be further damped by the interband transitions in the metal. For a spherically symmetric nanoshell, where the center of the inner shell surface is coincident with the center of the outer shell surface, plasmon hybridization only occurs between cavity and sphere plasmon states of the same angular momentum, denoted by multipolar index l (Δl=0). In the dipole, or electrostatic limit, only the l=1 dipolar bonding plasmon is excited by an incident optical plane wave. These selection rules allow for a limited number of transitions which result in a limited number of optical features. One method to increase the utility of these materials would be to increase the number of optical features associated with the nanoparticles. Thus it would be desirable to develop metallic nanostructures having enhanced optical properties.